1. Field of the Invention
The present invention pertains to an apparatus and method of providing substantially improved radiation beams for the treatment of surfaces, thin films, coatings, fluids or objects. More particularly, the present invention pertains to an apparatus and method for optically combining the light output of at least two arrays of solid-state light emitters to produce a light beam that has a selected spectrum chosen for applications requiring a wide range of wavelengths to improve or accelerate a treatment process with a controllable irradiance.
2. Description of the Prior Art
Radiant energy is used in a variety of manufacturing processes to treat surfaces, films, coatings, over layers, and bulk materials. Specific processes include but are not limited to curing, fixing, polymerization, oxidation, purification, or disinfections. By way of example, the manufacture of components for motor vehicles involves the application of under coatings, paints or clear coatings on vehicle surfaces for various purposes including corrosion resistance, decoration or surface protection (e.g. scratch resistance). The coatings or paints are resins or polymer-based materials that are applied as liquids or powders and require thermal or radiant energy processing to become solids. The processing of coatings or paints by thermal methods is slow and requires times ranging from minutes to hours to complete. In addition, some materials (for example, substrates or coating components) may be heat sensitive and damaged by thermal treatments.
Non-thermal curing using radiant energy to polymerize or effect a desired chemical change is rapid in comparison to thermal treatment. Radiation curing can also be localized in the sense that curing can preferentially take place where the radiation is applied. Curing can also be localized within the coating or thin film to interfacial regions or in the bulk of the coating or thin film. Control of the curing process is achieved through selection of the radiation source type, physical properties (for example, spectral characteristics), temporal variation, or the curing chemistry (for example, coating composition).
A wide variety of radiation sources are used for curing, fixing, polymerization, oxidation, purification, or disinfections of a variety of targets. Examples of such sources include but are not limited to photon, electron or ion beam sources. Typical photon beam sources include but are not limited to arc lamps, incandescent lamps, electrodeless lamps and a variety of electronic (that is lasers) and solid-state sources (that is solid state lasers, light-emitting diodes and diode lasers). Selection of a specific radiation source for an application is contingent on the requirements of the treatment process and the characteristics of the radiation source. These characteristics are related to but are not limited to the physical properties of the source, its efficiency, economics, or characteristics of the treatment process or target. For example, arc lamps or radio-frequency or microwave driven “electrodeless” ultra-violet sources efficiently produce high levels of radiated power having applications in many “industrial” processes where rapid treatment using significant levels of irradiance or energy density over large areas are needed. Arc or electrodeless lamps require high voltage, microwave or radio frequency power supplies and in the case of microwave-driven systems, a microwave tube (that is a magnetron). These high-powered lamps also require cooling and heat rejection systems. Such operational requirements limit the application of such photon sources to situations where this need can be met.
The spectral emissions of arc and electrodeless lamps are controlled by the conditions under which the lamp is operated, the particular gases used to fill the bulb and the selection of various additives placed in the bulb. Those skilled-in-the-art formulate specific lamp fills meeting curing needs for many photochemical processes, but gaps exist in spectral coverage in certain spectral ranges.
Solid-state light sources, such as, but not limited to, light emitting diodes (LEDs), diode lasers, diode pumped lasers and flash lamp-pumped solid-state lasers provide emission sources that can tuned to the needed wavelength or can be combined as arrays to provide a multi-wavelength source for applications needing broadband source. Advances in solid-state source technology provide high-brightness ultraviolet LEDs suitable as sources for radiation treatment.
At the present time, commercial UV emitting diodes emitting radiation down to an output of 370 nm. are available from Nichia, Cree, Agilent, Toyoda Gosei, Toshiba, Lumileds and Uniroyal Optoelectonics (Norlux).
UV emitting LEDs and laser diodes are constructed using large band gap host materials. InGaN based materials can be used in LEDs emitting at peak wavelengths ranging from 370 to 520 nm (for example, from the ultraviolet (UV-A) to visible green). The band gap of GaN is 3.39 eV and can accommodate luminescent transitions as large as 363 nm. The substitution of In into the GaN host provides localized states that can radiate in the ultraviolet down to 370 nm.
Other nitride materials such as InAlGaN can emit ultraviolet radiation in wavelengths as short as 315 nm. InAlGaN is already being used to make high brightness LEDs and laser diodes that operate in the range of 315 to 370 nm. Hirayama et. al (Appl. Phys. Lett. 80,207 (2002)) reports devices employing layered structures of InxGa1−xN or quaternary InxAlyGa1−x−yN grown on AlxGa1−xN (x=0.12−0.4) have been used in multiple quantum well structures to produce sources emitting comparable flux at 330 nm to InGaN devices operating at 415-430 nm. Hirayama et al. (Hirayama et al, Appl. Phys. Lett, 80, 1589 (2002)) has also reported a room temperature LED source using an improved multiple quantum well (MQW) structure and InAlGaN materials which emits intense UV radiation at 320 nm and significant emission at 300 nm.
Hirayama et al. (Appl. Phys. Lett. 80, 37 (2002)) report that AlxGa1−xN(AlN)/AlyGa1−yN MQWs exhibit efficient photoluminescence between 230 to 280 nm and that the photoluminescence is as high as that of the InGaN-based materials used in the violet diodes now commercially available. AlN-based materials are likely candidates for making ultraviolet LEDs operating in the UV-B or UV-C ranges. Other researchers are studying carbide and diamond materials as hosts for deep-UV based on the fact that their band gaps are as large as AlN.
LEDs operating in the blue, violet and UV-A (390 nm) wavelengths are of sufficient radiance to be used in ultraviolet and photochemical curing as “spot” curing sources. U.S. Pat. No. 6,331,111B1 (Cao) and EP 0-780-104 (Breuer et al) describe hand held portable spot curing light systems using solid state light sources consisting of light emitting diodes or diode laser chips. The light source of Cao may contain sources that emit multiple wavelengths so that numerous components in materials whose photo initiators are sensitive to different wavelengths may be cured at once. In the preferred embodiment described in Cao, the light travels directly to the curing surface without going through an optical device like a light guide or optical fiber. Breuer et al. describe a similar device optimized to cure dental resins and also extend claims to apparatus where the irradiator is a stationary curing apparatus whose light source chips are fixed to the walls of the curing chamber.
Various light sources have been used for the purposes of curing composite materials. These include plasma, halogen, fluorescent, and arc lamps. Various lasers have been incorporated in curing apparatus. Lasers emitting ultraviolet beams include frequency doubled or re-doubled sources like the 266 nm Nd-YAG systems, argon-ion systems and Nd-YAG pumped OPOs (optical parametric oscillators). Cao cites U.S. Pat. Nos. 5,420,768, 5,395,769, 5,890,794 and 5,161,879 where LEDs have been employed as curing light sources. The application of solid state sources to the curing process are also described in U.S. Pat. Nos. 6,127,447 and 5,169,675.
Technology necessary for the application of solid-state sources in the treatment process can be found in the development of LED and laser diode equipped systems for illumination and solid-state displays. These systems include an apparatus for LED illumination that can be incorporated into a hand-held lamp, are battery powered and equipped with electronics that provide pulsed power to control lamp radiance and compensate for the decrease in battery voltage during battery discharge. Published U.S. Patent Application 2002/0017844 A1 teaches the use of optical systems to modify the field of view for LED emitters in displays where the field-of-view is restricted.
There are many examples in the prior art of the use of LEDs in arrays to synthesize multi wavelength emissions. U.S. Published Patent Application No. 2001/0032985 A1 teaches the installation of arrays of colored LEDs on a chip to make multicolored or white solid-state illumination sources. U.S. Pat. Nos. 6,016,038 and 6,150,774 disclose the method and electronics needed to generate complex, predesigned patterns of light in any environment. The use of computer controlled LED arrays to provide light sources capable of rapid changes in illumination and spectral selection are detailed in U.S. Pat. No. 6,211,626, which describes a system using sub-arrays of primary colored (red, green and blue) LEDs whose individual elements are addressable and which can be controlled by pulse modulation to emit varying amounts of light to synthesize a third color. U.S. Pat. No. 6,211,626 indicates that such computer-controlled arrays of light emitters are not new but that previous systems had limitations, which reduced the flexibility or efficiency of the illumination system. The use of computer control for lighting networks used in illumination is described in U.S. Pat. Nos. 5,420,482, 4,845,481 and 5,184,114.
U.S. Published Patent Application No 2002/0191394 teaches the use of a diffractive optical element (diffraction grating) for mixing light from monochromatic light sources like LEDs and making multicolor or white beams. The monochromatic light sources are positioned relative to the grating where light of that frequency is found in the diffracted order beams higher than the zeroth order. The mixed beam is the zeroth order beam. A white beam will be provided if sufficient frequencies are represented in the first and higher order beams being directed on the grating. Fraunhoffer diffraction is used to mix the monochromatic beams. This is different from the use of Fresnel Zone plates to accomplish the coupling of the multiple radiation sources